In various mechanical devices, a need exists for a mechanical fuse or shear pin to sever a connection between two mechanical components when stress in excess of some pre-established amount occurs between the two parts. Shear pins, for example, are used to connect a propeller of an outboard motor to the drive shaft of the motor; so that if the propeller should strike an obstruction, the shear pin will break before damage to the drive train of the motor takes place. Typically, shear pins of this type are used to secure a propeller with a shaft-receiving portion on it by inserting the shear pin radially through a cylindrical extension on the propeller and through the drive shaft which is inserted into this extension. Thus, if an excessive rotational force should be generated between the propeller and the drive shaft, the shear pin is broken or "sheared"; and the drive shaft can spin free within the propeller extension without damage to the motor drive train.
Devices using shear pins which are oriented transversely to the direction of pull are disclosed in three United States patents to Smith U.S. Pat. No. 5,122,007, McCall U.S. Pat. No. 4,330,926, and Shearar U.S. Pat. No. 5,466,082. All of these patents employ essentially the same general principles of operation. The devices of the Smith and Shearar patents both are capable of relatively easy insertion and replacement of the shear pins. In the Smith device, the shear pin is held in place by means of a rubber "O" ring which encircles the device and extends across both ends of the shear pin. In the device of the Shearar patent, the shear pin has cotter pin retainers in both ends of it; so that a bit more effort is involved in replacing and changing the shear pin of this device than is required for that of the Smith device.
Some applications exist where a load fuse or a shear pin oriented in a longitudinal direction, or in the direction of pull, is preferred over the transversely oriented shear pins of the above patents. For example, for load lifting cables used with helicopters, it is desired to provide some type of link in the lifting cable which will break when an excessive load is applied to the cable. Lifting cables, which are used with helicopters to lift and deliver loads while the helicopter is airborne, are subject to situations where either the load or the cable become entangled with an obstruction. If the force exerted on the cable exceeds the load capabilities of the helicopter, a catastrophic accident may occur. Consequently, applications exist for an axial longitudinal mechanical load fuse for such situations.
The United States patent to Clifton U.S. Pat. No. 5,599,129 is directed to a load limited conductor using an axially oriented breakaway pin (or multiple pins). The pin or pins of this device are in the form of bolts which are used to interconnect the two connector portion of the assembly in which they are used. These bolts are simple threaded into place to interconnect the assemblies; and the bolts have a pre-established breaking tension. These devices are broken along their axial length or upon the application of axial tension to them.
The U.S. patent to Walker U.S. Pat. No. 3,185,415 is directed to an integral tension bar retainer for aircraft. The device of this patent employs a shear pin which is axially located within the retainer and is held in place by means of a spring-loaded retainer cam. Rotation of the cam allows insertion and replacement of different shear pins. There is nothing to prevent accidental rotation of the cam in this device.
It is desirable to provide a mechanical load fuse or shear pin device which overcomes the disadvantages of the prior art, which permits easy removal and insertion or replacement of different load fuses, which is simple to construct, and which functions to prevent accidental removal of a load fuse element in a completed assembly.